Machining head for a laser machining device

ABSTRACT

A processing head for a laser processing device adapted for processing a workpiece using laser radiation has: adjustable focusing optics to focus laser radiation in a focal spot having an adjustable distance from the processing head; an optical coherence tomograph to measure a distance between the processing head and the workpiece by measuring an optical interference between measuring light reflected by the workpiece and measuring light not reflected by the workpiece; a path length modulator that can change, synchronously with and dependent on a change of the focal spot distance from the processing head, an optical path length in an optical path along which measuring light propagates; a scanning device, which deflects the laser radiation in different directions; and a control device, which i) controls a focal length of the focusing optics in such a way that the focal spot is situated at a desired location on the workpiece, ii) receives, from the coherence tomograph, information representing the distance between the processing head and the workpiece, and iii) uses information received from the coherence tomograph for a continuous correction of a positioning of the focal spot on the workpiece.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority to and thebenefit of, co-pending U.S. application Ser. No. 14/938,398 filed onNov. 11, 2015 and entitled “Machining Head for a Laser MachiningDevice”, which is a continuation of International Patent ApplicationPCT/EP2014/001234 filed May 8, 2014 and which claimed priority to andthe benefit of German patent application No. 10 2013 008 269.2 filed May15, 2013. The full disclosures of these earlier applications are herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a processing head for a laser processingdevice, with which workpieces can be welded, cut or otherwise processed.

2. Description of the Prior Art

Laser processing devices normally comprise a laser radiation sourcewhich may, for example, be a fibre laser or a disc laser. A laserprocessing device further includes a processing head, which focuses thelaser radiation generated by the laser radiation source in a focal spot,and a beam supply device, which supplies the laser radiation generatedby the laser radiation source to the processing head. The beam supplydevice may comprise optical fibres or other optical waveguides and/orone or more deflecting mirrors with plane or curved surfaces. Theprocessing head can be fastened to a movable robot arm or another movingdevice which enables a three-dimensional positioning. The laserradiation source is often arranged further away from the processing heador a moving device carrying the latter.

Hitherto the workpieces to be processed have usually been brought into adefined position by handling devices. The processing head is then guidedwith the aid of the robot at a distance of a few centimetres above thestationary workpiece. Since the processing head is heavy, it is notpossible to perform very fast movements, as would be appropriate forexample in spot welding operations. In principle, the workpiece couldadditionally be moved, but this increases the constructional expenditureon the handling devices.

In order to be able to process workpieces quickly at locations lying farapart, laser processing devices have therefore been developed in whichthe focal spot of the laser radiation is guided with the aid of ascanning device, which usually includes an arrangement ofgalvanomirrors, over the workpiece. If the processing head is far enoughaway (e.g. about 50 cm) from the workpiece, locations lying far apart onthe workpiece can be processed extremely quickly by the laser radiation.The movement of the relatively heavy processing heads is thus replacedby movements of the light galvanomirrors in the scanning device.Processing methods in which the processing head is situated far awayfrom the workpiece and include a scanning device are often called remotelaser welding (or welding-on-the-fly) or remote laser cutting.

Besides the higher processing speed, these methods have the advantagethat spatter and other contamination arising during the processing canhardly reach and contaminate the processing head any more. Protectiveglasses on the processing head thus need to be replaced less frequently,thereby reducing the downtimes. Moreover, the processing head no longerneeds to be moved at all or if necessary moved only relatively slowly,making a robot redundant or more cost-effective.

A problem when using such laser processing devices which has hithertonot yet been satisfactorily solved is that it is difficult to positionthe focal spot, the diameter of which in welding processing mostly liesbetween 100 μm and 500 μm and in cutting processing may be 20 μm andless, precisely on the surface of the workpieces to be processed.Therefore, it has hitherto not been possible, for example, to producefillet weld seams at lap joints of galvanised steel plates, because thefocal spot cannot be positioned accurately enough in the fillet weld.For this reason, hitherto galvanised steel plates have mostly beenjoined by a laser deep welding process, in which an air gap must be leftbetween the surfaces. This air gap is necessary so that the zinc coatingmelted on in an explosive manner can spread. The formation of cavitiesand defects along the weld seam can thereby be prevented. In order tokeep the steel plates at a distance, they must have distance-maintainingindentations. The difficulty in reliably producing fillet weld seamsthus ultimately leads to restrictions in the design of the workpiecesand additional material consumption.

The reasons why the focal spot cannot be positioned accurately enough onthe workpieces to be processed are as follows. Hitherto in remote laserprocessing the focal spot has been guided over the workpieces to beprocessed in accordance with a predetermined control program. Shapedeviations of the workpieces themselves and positioning tolerances ofthe handling devices and optionally used robot, however, result in thelocation to be processed on the workpiece often not being situated atits desired position. Since such deviations are not taken into accountin the control, the processing actually takes place outside the desiredposition.

It would be ideal if the focal spot could track the actually encounteredspatial arrangement of the workpieces in a regulating process. For thispurpose, however, it would be necessary to measure this actual spatialarrangement of the workpieces to be processed relative to the processinghead or another reference point during the laser processing in realtime. However, it has not been possible hitherto to carry out such ameasurement successfully.

An observation of the processing location with the aid of a camera doesnot therefore lead to the desired improvements, because the cameracaptures only a 2D projection of the workpieces. If the beam path of thecamera runs coaxially with the laser radiation, as is known in the priorart, although a lateral offset along the directions X and Y can bemeasured accurately, the distance between the workpiece and theprocessing head along the Z direction cannot be measured accurately.Because for high process quality, measuring accuracies are required inthe Z direction which are in the order of around 400 μm for weldingprocessing and in the order of around 100 μm for cutting processing.

For light-section or triangulation methods the distance between theprocessing head and the workpiece is too great to be able to measurewith sufficient precision.

Chromatic-confocal measuring methods are also unsuitable, because on theone hand the numerical aperture of the focusing optics in the processinghead is too low and on the other hand the chromatic longitudinalaberration thereof is too small to be able to cover a sufficientmeasuring range.

For distance measurement during the laser processing, some time ago theuse of optical coherence tomographs (OCT) was proposed, cf. inparticular EP 1 977 850 B1, DE 10 2010 016 862 B3 and DE 10 2012 207 835A1. Optical coherence tomography enables high-precision distancemeasurement and even the generation of a 3D profile of the scannedsurfaces when the measuring beam is guided scanner-like over thesurfaces.

For remote laser processing in which the distance between the focal spotand the processing head can vary in the Z direction by up to 50 cmwithin fractions of a second, the optical coherence tomographs known inthe prior art are, however, not suitable. Coherence tomographs whichoperate in the time domain (TD-OCT) usually contain a mirror in thereference arm of the coherence tomograph which modulates the opticalpath length thereof. The mirror vibrates at high frequency in the axialdirection, whereby depth information can be obtained sequentially. Themoving distance covered by the movable mirror is, however, only in theorder of a few millimetres. The measuring range of such TD-OCTs is thuslikewise only a few millimetres and would thus be a good two orders ofmagnitude too small for remote laser processing.

Coherence tomography in the frequency domain (FD-OCT), in which theoptical path length in the reference arm is not changed, can alsoachieve a measuring range of only a few centimetres. For conventionallaser processing devices in which the processing head is guided at anapproximately constant distance over the workpieces, this measuringrange is perfectly adequate. For remote laser processing, however, thismeasuring range is also insufficient.

From US 2012/0138586 A1 there is known a laser processing device havingan OCT, in which the optical path length in a reference arm of the OCTcan be tracked when the focal spot of the measuring beam is laterallydeflected.

In DE 102 02 036 A1 there is described a laser processing device havinga deflecting device and adjustable focusing optics. The focal spot ofthe laser radiation can thereby be moved in a manner which is plane andperpendicular to the beam direction.

SUMMARY OF THE INVENTION

The object of the invention is to specify a processing head of a laserprocessing device, with which large and especially greatly varyingdistances from a workpiece can also be precisely measured.

According to the invention this object is achieved by a processing headfor a laser processing device, which is adapted for the processing of aworkpiece using laser radiation, the processing head having adjustablefocusing optics, which are adapted to focus the laser radiation in afocal spot, the distance between the focal spot and a processing headbeing changeable by changing the focal length of the focusing optics.The processing head may comprise a scanning device, which is adapted todeflect the laser radiation in different directions. The processing headfurther has an optical coherence tomograph, which is adapted to measurea distance between the processing head and the workpiece along a singledirection and/or along different directions. Measuring light, which hasbeen generated by a measuring light source and reflected by theworkpiece, interferes in the coherence tomograph with measuring lightwhich has travelled an optical path length in a reference arm. Accordingto the invention, there is arranged in the reference arm a path lengthmodulator which tracks the optical path length in the reference armsynchronously with and dependent on a change of the focal length of thefocusing optics.

Through the synchronous tracking of the optical path length in thereference arm, the axial measuring range of the coherence tomograph canbe increased in fact to almost any size. If, for example, the focallength of the focusing optics is changed such that the focal spot shiftsby 30 cm away from the processing head, the path length modulatorincreases the optical path length in the reference arm synchronously bythe same amount. Then for the new distance value the normal measuringrange of the coherence tomograph is available again, which in the caseof coherence tomographs in the frequency domain (FD-OCT) should notexceed about 8 mm, in order to maintain sufficient measuring accuracy.Since the coherence tomograph per se can measure only differencesbetween the optical path lengths in the object arm and in the referencearm, the optical path length added in the tracking by the optical pathlength modulator is to be taken into account when calculating the actualdistance between the workpiece and the processing head. The sameapplies, of course, conversely also in the case where the distancebetween the focal spot and the processing head is reduced.

In general, the control of the path length modulator is performed suchthat when the focal length of the focusing optics changes by Δd, thepath length modulator changes the optical path length in the referencearm by 2Δd. The factor 2 results from the fact that the measuring lightguided in the object arm is also reflected and thus travels the focallength of the focusing optics twice. In principle, deviations from thiscondition are permissible and may also be appropriate in individualcases. For example, when changing from a processing location which issurrounded by elevated structures to a processing location which issurrounded by sunken structures, it may be expedient to depart from theaforementioned principle in order to be able to utilise the measuringrange of the coherence tomograph optimally.

In principle, the invention is also applicable in coherence tomographswhich operate in the time domain (TD-OCT). In this case, the opticalpath length modulator must additionally generate a high-frequency pathlength modulation with low path length stroke in the order of a fewmillimetres. But for application in remote laser processing, as alreadymentioned above, coherence tomographs in the frequency domain (FD-OCT)are generally more favourable, since they can cover a greater axialmeasuring range.

In remote laser processing, the focal length of the focusing optics canchange by greater amounts within fractions of a second. At the sametime, the measuring range of the optical coherence tomograph must alsobe able to be shifted by the same distance with the aid of the pathlength modulator. With the path length modulators described below, it ispossible to produce smaller changes of the optical path length (OPD=20mm) in less than 10 ms. Likewise it is possible to produce somewhatlarger changes of the optical path length (OPD=100 mm) in less than 20or greater changes of the optical path length (OPD=200 mm) in less than50 ms.

In order to produce such large changes of the optical path length insuch a short time, the path length modulator must not contain any largermasses which have to be moved quickly. Therefore, conventional pathlength modulators which comprise a linearly movable mirror in a beampath folded twice are too slow.

In one group of exemplary embodiments, the path length modulatortherefore has a displaceable mirror, which is arranged in a folded beampath in such a way that on a shifting of the mirror by the distance sthe optical path length in the reference arm changes by at least 8s, andpreferably by at least 12s and further preferably by at least 16s. Bysuch folding of the beam path, with relatively short axial shifts of themirror large changes of the optical path length in the reference arm canbe produced. Compared with conventional path length modulators in thereference arm, in which on a shifting of the mirror by the distance sthe optical path length changes only by 2s, a compression of the beampath by a factor of at least 4 is therefore achieved. Here the sum ofthe optical path lengths on the outward and return path is regarded asthe optical path length in the reference arm.

A 6-fold compression of the beam path can be produced, for example, whenthe path length modulator has an optical axis and two pairs ofreflecting plane surfaces which are respectively arranged at an angle of90° to one another and at 45° with respect to the optical axis. Thepairs are then to be arranged in a manner rotated azimuthally withrespect to the optical axis by an angle of 60° to one another. Anazimuthal rotation by an angle of 45° results in an 8-fold compression,and a rotation by an angle of 30° even results in a 12-fold compression.

In another group of exemplary embodiments, the path length modulator hasa multiplicity of optical channels metal-coated at the end side and ofdifferent length and an optical switch by which the measuring light canbe sequentially coupled into respectively one of the optical channels.By such a path length modulator the optical path length is thus changednot continuously, but stepwise. The optical channels can in this caseeach have a free space, through which the measuring light can propagate,and comprise a reflecting surface.

Optical channels of different length can be produced in a particularlyspace-saving manner if they are formed as optical fibres. The opticalfibres can be space-savingly rolled up or otherwise bent, so that thechannels of different length can be easily accommodated even in theconfined spatial conditions of a processing head.

The optical switch may, for example, be a movably mounted mirror whichis optionally curved, in order for example to be able to couplemeasuring light into optical fibres. However, fibre-optic or integratedspatial multiplexers, as known from optical communication technology,also come into consideration as optical switches.

With the processing head according to the invention, it is possible forthe processing head to be assigned a regulating device which is adaptedto regulate the focal length of the focusing optics, and/or a directionof the laser radiation set by the scanning device, in such a way thatthe focal spot is situated at a desired location on the workpiece, theregulating device being able to be supplied with the distance, measuredby the coherence tomograph, between the processing head and theworkpiece.

In general, it will be preferred if the measuring light passes throughat least one part of the focusing optics, by which its focal length canbe changed.

When changing the focal length of the focusing optics, the focal spot ofthe measuring light is thus always automatically moved along therewith.At the same time, at least parts of the focusing optics can be used tofocus the measuring light. Because only sufficient focusing of themeasuring light, which is preferably less than four times the Rayleighlength, ensures a good quality of the distance measurement with thecoherence tomograph.

Through the partial joint use of the focusing optics, it can be ensured,moreover, in a particularly simple manner, that the measuring light isalways focused in the same focal plane in which the focal spot of thelaser radiation is also situated. For, in general, it is particularlyimportant to know how far the workpiece is away from the processing headat the processing point at which the focal spot of the laser radiationis formed. The measuring light generated by the coherence tomograph doesnot necessarily have to be directed at the focal spot. Thus, forexample, it is possible to provide for the measuring light an additionalscanning device which travels scanner-like over a region surrounding theprocessing point on the workpiece and in this way provides athree-dimensional relief of the surface. It is also possible for themeasuring light beam to travel on a circular path around the processingpoint. Such a circular scanner arranged in the optical path of themeasuring light can be realised very easily with the aid of a wobblemirror, a micromirror tiltable about two tilt axes or a rotating wedgeplate. The information obtained by a circular scanner is oftensufficient for seam tracking.

The coherence tomograph can thus be used in this case not only toposition the focal spot optimally on the workpiece, but also forpurposes of subsequent seam tracking. In this way, for example, cavitiesand other defects can be identified in the course of the qualityassurance.

In principle, it is also possible to provide for the measuring lightdedicated focusing optics and a dedicated scanning device, throughneither of which the laser radiation passes. For workpieces with a verypronounced height profile, it is thus possible to perform a highlyaccurate surface measurement in the wider surroundings of the processingpoint, since the measuring light and the laser radiation no longer haveto be focused in the same focal plane.

Fundamentally, however, the invention can also be used in laserprocessing devices which do not contain a scanning device. In this case,only the shifting of the focal spot in the axial direction is performedwith the aid of the focusing optics, while the lateral shifting of thefocal spot is produced by moving the processing head. The subject-matterof the invention is furthermore a method for laser processing of aworkpiece with laser radiation, the method comprising the followingsteps:

-   a) focusing the laser radiation in a focal spot, the distance    between the focal spot and a processing head being changed by    changing the focal length of focusing optics included in the    processing head;-   b) using a coherence tomograph, which includes a reference arm, for    measuring the distance to the workpiece;-   c) tracking the optical path length in the reference arm    synchronously with and dependent on the change of the focal length    in step a).

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom the following description of the exemplary embodiments with the aidof the drawings, in which:

FIG. 1 shows a schematic representation of a laser processing deviceaccording to the invention during the welding of two steel plates alonga fillet weld;

FIGS. 2 and 3 show the beam path in a processing head of the laserprocessing device shown in FIG. 1 for two different focal spotpositions;

FIG. 4 shows a meridional section through a path length modulator with abeam path folded twice;

FIG. 5 shows a variant of the path length modulator shown in FIG. 4;

FIG. 6 shows a path length modulator according to the invention with abeam path folded 4 times, in a perspective representation;

FIG. 7 shows front views of prisms of the path length modulator shown inFIG. 6, on which the points of incidence of the light beams are marked;

FIGS. 8 to 10 show points of incidence on prisms or reflecting surfaceswith a beam path folded 6 times, 8 times and 12 times, respectively;

FIGS. 11a and 11b show a path length modulator according to theinvention, in which an optical switch couples measuring light intooptical fibres of different length, in two different switchingpositions;

FIGS. 12a and 12b show a variant of the exemplary embodiment shown inFIGS. 11a and 11b , in which the measuring light propagates not inoptical fibres, but in free space;

FIGS. 13a and 13b show a part of the beam path shown in FIG. 2, withadditionally a wobble mirror being provided for generating a measuringlight beam circulating on a circular path;

FIGS. 14a and 14b show a detail of a beam path in a processing headaccording to another exemplary embodiment, in which two lenses are movedin the beam path of the laser radiation and of the measuring lightjointly for the axial shifting of the focal spot;

FIG. 15 shows a graph in which measuring signals of the coherencetomograph are plotted schematically.

DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS 1. Structure of the LaserProcessing Device

FIG. 1 shows in a schematic representation a laser processing device 10having a robot 12 and a processing head 14 according to the invention,which is fastened to a movable arm 16 of the robot 12.

The laser processing device 10 also includes a laser radiation source 18which is formed in the illustrated exemplary embodiment as a disc laseror fibre laser. Laser radiation 30 generated by the laser radiationsource 18 is supplied via an optical fibre 20 to the processing head 14and focused by the latter in a focal spot 22.

The laser processing device 10 is provided for a remote laser processingof workpieces. The distance between the focal spot 22 and the processinghead 14 is therefore about 30 cm to 100 cm. As will be explained belowwith reference to FIG. 2, the focal length of focusing optics includedin the processing head 14 is changeable in order to be able to positionthe focal spot 22 on the workpiece at different distances from theprocessing head 14. In addition, the processing head 14 includes ascanning device, with which the laser radiation can be deflected indifferent directions. In this way, it is possible to position the focalspot 22 at any desired point within a larger volume.

In the exemplary embodiment illustrated in FIG. 1, the workpieces aretwo galvanised steel plates 24 a, 24 b which are held in a particularrelative position to one another with the aid of handling devices 26 aand 26 b, respectively. It is further assumed that the laser processingdevice 10 is to weld a fillet weld at a lap joint 28 between the twosteel plates 24 a 24 b. The control of the laser processing deviceensures that the robot 12 and thus the processing head 14 fastenedthereto move only slowly or not at all while the focal spot 22 is guidedalong the lap joint 28. The scanning device and the focusing optics arecontrolled such that the focal spot 22 moves relative to the processinghead 14 and in doing so travels on the surface of the steel plates 24 a,24 b along the lap joint 28. During this, an optical coherence tomograph48 integrated in the processing head 14 continuously measures thedistance between the processing head 14 and the steel plates 24 a, 24 balong the current direction of the laser radiation 30.

FIG. 2 shows the beam path in the processing head 14 in a schematicrepresentation. It is assumed that the laser radiation 30 enters theprocessing head 14 already collimated. The laser radiation 30 firstlypasses through a dichroic first beam splitter 32 and then the focusingoptics, designated by 34, of the processing head 14. The focusing optics34 comprise a diverging first lens 36 which can be moved along anoptical axis 38 with the aid of a moving device 40, as indicated by adouble-headed arrow above the first lens 36. The focusing optics 34 alsocomprise a converging second lens 42 which is fixedly arranged. Bymoving the first lens 36, the focal length of the focusing optics 34 andthus the distance from a light exit window of the processing head 14 canbe changed.

In the beam path downstream of the focusing optics 34, the scanningdevice, already mentioned and designated by 44, is arranged. This deviceincludes one or more galvanically suspended mirrors, with which thelaser radiation 30 can be deflected in different directions, asindicated by a dashed beam path 46. Since such scanning devices areknown as such in the prior art, a more detailed explanation of thestructure is omitted.

Through the interaction of the scanning device 44 with the moving device40 of the focusing optics 34, the focal spot 22 can be positioned at anydesired locations on the steel plates 24 a, 24 b within a larger volume.

In order to be able to measure the distance of the workpiece from theprocessing head 14, the latter includes the optical coherence tomographalready mentioned, which is designated as a whole by 48 in FIG. 2. Thecoherence tomograph 48 comprises a light source 50 for generatingmeasuring light 52, a second beam splitter 54 and a third beam splitter56, downstream of which the beam path divides into an object arm 58 anda reference arm 60.

In the object arm 58 the measuring light is firstly widened bytelescopic optics 62 and then coupled into the beam path of the laserradiation 30 by the first beam splitter 32. The telescopic optics 62comprise, in the exemplary embodiment illustrated, a diverging lens 64and a converging lens 66. Arranged between the lenses 64, 66 is anadjustable glass path adapter 68, consisting of two wedge prisms 70, 72which are displaceable relative to one another and between which anindex-matched gel is situated. Through displacement of the wedge prisms70, 72, the axial length of the glass path adapter 68 can becontinuously adjusted. In this way, it is possible to integrate theoptical coherence tomograph 48 also into existing processing heads 14and, with the aid of the glass path adapter 68, always set equal pathlengths in dispersing glasses.

Situated at the end of the reference arm is a path length modulator 74consisting, in the exemplary embodiment illustrated, of a plane mirror76 which is movable in the axial direction with the aid of a lineardrive 78. In this way, the optical path length in the reference arm 60can be continuously adjusted.

The coherence tomograph 48 further comprises a spectrally resolvinglight sensor 80 which detects the interference of measuring light, whichhas been reflected by the steel plates 24 a, 24 b, with measuring lightwhich has travelled a similar optical distance in the reference arm.

The coherence tomograph 48 operates in the frequency domain (FD-OCT). Asa result, for a given optical path length in the reference arm 60, ameasuring range in the axial direction of about 8 mm is possible. Since,apart from the path length modulator 74 which has yet to be explained inmore detail, such coherence tomographs 48 are known in the prior art,the function will not be explained again in detail here. Fundamentally,coherence tomographs with optical circulators, as described in DE 102010 016 862 B3 of the applicant, are also suitable.

FIG. 3 corresponds to FIG. 2, except that there the focal spot 22 wasshifted in the axial direction. For this purpose, the first lens 36 ofthe focusing optics 34 was displaced by the moving device 40 such thatthe focal length of the focusing optics 34 lengthens.

If this lengthening goes beyond about 8 mm, the measuring range of thecoherence tomograph 48 would thereby be exceeded. Therefore, the planemirror 76 in the path length modulator 74 is shifted synchronously withthe displacement of the first lens 36 by the same amount by which thefocal length was lengthened. In this way, the optical path lengths inthe reference arm 60 and in the object arm 58 correspond again, so thatthe entire measuring range of the coherence tomograph 48 of about 8 mmis usable.

In order to synchronise the displacements of the lens 36 of the focusingoptics with the plane mirror 76 of the path length modulator 74, themoving device 40 and the path length modulator 74 are connected to acommon regulating and control device 82 via signal lines. The regulatingand control device 82 also controls the scanning device 44. It issupplied with measuring signals of the light sensor 80 which, afterevaluation, provide the optical path length difference in the object armand the reference arm 58 and 60, respectively. Since the optical pathlength in the reference arm is known (the axial position of the planemirror 76 should therefore be detected exactly by an encoder), theregulating and control device 82 can control the moving device 40 of thefocusing optics 34 and also the scanning device 44 such that the focalspot is positioned exactly at the desired location on the steel plates24 a, 24 b. This positioning is preferably carried out by means of aregulating loop, in which the measured values provided by the coherencetomograph 48 are used for continuous correction of the positioning ofthe focal spot 22.

2. Path Length Modulators

For workpieces with a large depth profile, the focal spot 22 must beshifted very rapidly by greater distances in the axial direction. Forthis there suffice short moving distances of the first lens 36 of thefocusing optics 34, which are easy to accomplish in spite of therelatively large mass of the first lens 36. The axial shifting of thefocal spot 22 can in this case be greater, for example, by an order ofmagnitude than the moving distance of the first lens 36.

The situation is different, however, in the path length modulator 74. Inthe exemplary embodiment shown in FIGS. 2 and 3, the beam path of themeasuring light 52 in the reference arm 60 is folded twice. This meansthat with a moving distance Δd of the plane mirror 76, the optical pathlength changes by the amount 2Δd. If, for example, the axial position ofthe focal spot 22 shifts by 200 mm in 30 ms, the plane mirror 76 must bemoved in this short period of time by 100 mm with the aid of the lineardrive 78. This requires extremely high accelerations of the plane mirror76.

a) Beam Folding

In the exemplary embodiment of the path length modulator shown in FIG.4, the optical path of the measuring light 52 is therefore compressed bymultiple folding not only twice, but four times. The plane mirror 76 inthis exemplary embodiment is replaced by a 90° prism 84, the hypotenusesurface of which is arranged perpendicular to the direction of incidenceof the measuring light 52. The 90° prism 84 therefore deflects themeasuring light 52 in a parallel-offset manner and directs it onto afixed plane mirror 86. The measuring light 52 then travels along thebeam path in the opposite direction, so that the distance between thefixed plane mirror 86 and the 90° prism 84 is travelled a total of fourtimes by the measuring light 52.

A shifting of the 90° prism 84 in the axial direction, as indicated inFIG. 4 by the double-headed arrow 88, has the result, with such a 2-foldcompression of the beam path, that the optical path length of themeasuring light 52 is changed by 4 times the moving distance of the 90°prism 84. Compared with the exemplary embodiment shown in FIGS. 2 and 3,adjusting times approximately half as long are thereby made possiblewhile less constructional space is taken up.

Especially when the measuring light 52 is guided in the coherencetomograph 48 not in free space, but in optical fibres, the variant of apath length modulator 74 shown in FIG. 5 may be expedient. The measuringlight 52 emerging slightly divergently at a fibre end 90 isparallel-offset by the 90° prism 84 and directed at a plane mirror 86.In this variant, however, between the 90° prism 84 and the plane mirror86 there is situated a converging lens 92 which maps the fibre end 90onto the plane mirror 86. In this way, it is ensured that the measuringlight 52 divergently emerging from the fibre end 90 is completelycoupled into the fibre end 90 again, after passing through the pathlength modulator 74.

FIG. 6 shows another exemplary embodiment of a path length modulator 74,in which two 90° prisms 84, 94 are arranged rotated by 90° to oneanother azimuthally, i.e. with respect to the Z-axis. Each of thereflecting surfaces of the 90° prisms 84, 94 thereby form an angle of45° with the optical axis (Z-axis). This enables a 4-fold compression ofthe beam path. A measuring light beam 1 entering at “In” is reflected,in the exemplary embodiment shown in FIG. 4, in the YZ-plane, by thefirst 90° prism 84 in a parallel-offset manner (cf. measuring light beam2). The second 90° prism 94 brings about a parallel offset in thevertical direction, i.e. in the XZ-plane (cf. measuring light beam 3).After a further horizontal offset in the first 90° prism 84, themeasuring light 52 is directed as measuring light beam 4 at “Out” by aplane mirror 85 or a prism surface onto a converging lens 87 and afurther plane mirror 89. Then the measuring light 52 travels over theabove-explained beam path again in the reverse direction. By displacingthe first 90° prism 84 in the axial Z-direction indicated by adouble-headed arrow, the optical path length is changed here by 8 timesthe amount of the displacement distance. This corresponds to acompression of the beam path by a factor of 4 compared with a singlereflection in the beam path of the reference arm, as known fromconventional coherence tomographs.

This structure too can be combined with the principle shown in FIG. 5,according to which a fibre end 90 is mapped onto a reflecting surface.

FIG. 7 shows in a schematic representation the piercing points of themeasuring light beam 1 to 4 shown in FIG. 6 on the hypotenuse surfacesof the two 90° prisms 84, 94. A circle with a dot in the middle denotesa measuring light beam entering on the first pass (i.e. before thereflection on the plane mirror 89), while a circle with a cross in themiddle indicates an emerging measuring light beam. The dashed linesindicate planes of symmetry of the 90° prisms 84, 94.

If the two 90° prisms 84, 94 are arranged at an azimuthal angle of 60°to one another, as illustrated in a representation based on FIG. 7, a6-fold compression of the beam path can thus be realised. In anarrangement at an azimuthal angle of 45°, as shown in FIG. 9, even an8-fold compression of the beam path can be achieved. Of course, thereflecting surfaces must not be surfaces of a prism. In order, forexample, to be able to arrange an axis of the linear drive, it may beexpedient to form at least some of the surfaces inclined by 45° withrespect to the optical axis as normal plane mirrors. FIG. 10 shows anarrangement for a 12-fold folding of the beam path, in which the second90° prism 94 has been changed into an arrangement of two plane mirrorpairs 94 a, 94 a′ and 94 b, 94 b′.

b) Optical Switches

In the second group of path length modulators, the optical path lengthin the reference arm 60 is not continuous, but changes in discretesteps. For this purpose, the path length modulator 74 has a multiplicityof optical channels metal-coated on one side and of different length andan optical switch, with which the measuring light 52 can be sequentiallycoupled into respectively one of the optical channels.

In the exemplary embodiment shown in FIGS. 11a and 11b , the opticalchannels are formed as optical fibres 96-1 to 96-8. A reflecting surfaceat the end of each fibre 96-1 to 96-8 is indicated by 98. In thesimplest case, the reflecting surface is a metal-coated end surface ofthe optical fibre. To avoid polarisation dependencies, a so-calledFaraday mirror can be attached to an antireflection-coated end surfaceof the fibre. A Faraday mirror consists of a collimator lens, abirefringent plate which rotates the polarisation direction by 45°, anda plane end mirror. A double rotation of the polarisation device by 45°then also has to be carried out in the object arm 58.

Each of the total of 8 optical fibres 96-1 to 96-8 has a differentlength. The optical switch is formed as a rotatably mounted tiltingmirror 100. Measuring light 52 entering the reference arm 60 is coupledby the tilting mirror 100 into an antireflection-coated end surface ofone of the optical fibres; in FIG. 11a this is the fibre 96-2. Afterpassing through the optical fibre 96-2 and reflection at themetal-coated end surface 98, the measuring light 52 emerges from theopposite antireflection-coated end surface and is directed by thetilting mirror 100 back in the direction of the third beam splitter 56again.

Through pivoting of the tilting mirror 100, the measuring light 52 canbe coupled into any of the eight optical fibres 96-1 to 96-8. In eachoptical fibre 96-1 to 96-8, the measuring light 52 travels a differentoptical distance.

FIG. 11b illustrates the case where the tilting mirror 100 has beentilted by control by regulating and control device 82 such that themeasuring light 52 is coupled into the optical fibre 96-7. As a result,the optical path length increases abruptly.

The optical fibres 96 have the advantage that they can be rolled up orotherwise space-savingly arranged in the processing head 14. As aresult, optical path length differences of practically any size can beproduced.

In the exemplary embodiment of a path length modulator 74 shown in FIGS.12a and 12b , the same principle is transferred to a free-spacepropagation. The optical fibres 96 are omitted here; instead collimatedmeasuring light 52 is directed from the tilting mirror 100 to one ofseveral plane mirrors 102-1 to 102-8.

The plane mirrors 102 have different distances from the tilting mirror100 and are so oriented that incident measuring light 52 is alwaysreflected back on itself. As FIG. 12b shows, the optical distance can beabruptly changed by a greater amount here too by pivoting the tiltingmirror 100.

In order to make the arrangement less sensitive to adjustmenttolerances, a converging lens 103 is respectively arranged in the beampath between the tilting mirror 100 and the plane mirrors 102-1 to102-8. This lens focuses the incident measuring light 52 to a point onthe respective plane mirror 102-1 to 102-8.

If the measuring light 52 emerges from an optical fibre, it can becollimated with the aid of a converging lens before impinging on thetilting mirror 100, as shown in FIG. 5.

3. Further Exemplary Embodiments

FIGS. 13a and 13b show an alternative exemplary embodiment of aprocessing head according to the invention in illustrations based onFIG. 2. The measuring light 52 here is not focused in the focal spot 22of the laser radiation 30, but moves on a circular path around the focalspot 22. For this purpose, the measuring light 52 emerging from theobject arm 58 of the coherence tomograph 48 is directed, even beforecoupling into the beam path of the laser radiation 30, by a deflectionmirror 104 onto a plane wobble mirror 106, which is wobblingly mounted.As a result, a surface normal of the wobble mirror 106 passing throughthe wobble axis 105 describes the path of a right circular cone.Consequently, the focal spot 108 of the measuring light 52 also moves atleast approximately on a circular path around the focal spot 22 of thelaser radiation 30. Two opposite positions of the measuring light focalspot 108 are shown in FIGS. 13a and 13b . The angle of rotation of thewobble mirror 106 with respect to the wobble axis 105 differs here by180°.

An approximately circular scanning of the surface of the workpiece 24 tobe processed is not suitable for regulation of the focal spot distance,but also allows a detection of the seam following the laser processing.Processing errors can thereby be detected early. The circular shape hereensures an independence from the processing direction. The circularradius here can be in the order of about 5 mm. Then there is stillenough time, depending on the measuring results, to intervene in theprocessing procedure.

FIGS. 14a and 14b show another alternative exemplary embodiment of aprocessing head according to the invention in meridional sections fortwo different focal lengths of the focusing optics 34. In contrast tothe exemplary embodiment shown in FIG. 2, the measuring light 52 iscoupled into the beam path of the laser radiation 30 not before, butonly within the focusing optics 34. In contrast to the exemplaryembodiment shown in FIG. 2, in the focusing optics 34 the divergingfirst lens 36 is replaced by a converging first lens 36′. The convergingeffect is required, since in this exemplary embodiment the laserradiation 30 exits divergently from a fibre connector 110.

The measuring light guided in the measuring arm 58 of the coherencetomograph 48 likewise exits from an optical fibre 112 and thereforelikewise passes through a converging lens 114 before it is coupled via adeflecting mirror 116 into the focusing optics 34.

In order that the focal spot 108 of the measuring light 52 is alwayssuperimposed on the focal spot 22 of laser radiation, the converginglens 114 must also be shifted in the beam path of the measuring light52, since the linearly-movably arranged converging first lens 36′ istraversed only by the laser radiation 30. In the exemplary embodimentillustrated, the converging lens 114 is therefore moved by the samemoving device 40 as the converging first lens 36′. It is even possibleto arrange the two lenses 36′, 114 on the same moving carriage, so thatthey move synchronously and at the same speed.

4. Planning of the Processing Procedure

When planning the processing procedure it should be taken into accountthat the measuring range of the coherence tomograph 48 consists of twohalves, which are interrupted by a dead zone in the middle. The reasonfor this is that the measuring range of FD-OCTs does not begin with apath difference of 0, but where a significant modulation can already beseen. The upper boundary of the measuring range, however, is reached onarriving at the undersampling.

Without any special measures, it is indistinguishable whether thereference arm 60 or the object arm 58 is longer, i.e. whether themeasured optical path length difference is negative or positive. This isillustrated by FIG. 15, which shows a graph in which, by way of exampleand schematically, the measuring signals of the coherence tomograph 48are plotted for the case where a measurement object transparent to themeasuring light 52 is measured, comprising two layers 120, 122 ofdifferent thickness. The virtual position of the reference plane, whichis determined by the optical path length in the reference arm 60, isdenoted by 124. At each boundary surface of the measurement object,measuring light is reflected. By interference with the measuring lightfrom the reference arm, there is formed in the evaluated depth slice,which is also referred to as an A-scan, a respective distance peak foreach boundary surface. The two distance peaks in the “−” OCT measuringsubrange have an inverted sign and thus appear in the A-scan of theFD-OCT at mirrored positions. In the evaluation it must therefore beclear from the context which of the measured distances have a negativesign with respect to the reference plane 124.

If the workpieces can be positioned with the precision of the measuringrange of the coherence tomograph (about 8 mm), then the processingprocedure can be set, for example, so that the optical path length inthe reference arm 60 is initially set by the path length modulator 74 sothat the distance on travelling along the processing line remains safelyin one of the two OCT measuring subranges “+” or “−” shown in FIG. 15.

In the case of heavily stepped objects, the measuring range can bestarted so that one of the two steps remains in the “−” OCT measuringsubrange and the other step remains in the “+” OCT measuring subrange.The transition between upper and lower step must then be recognised fromthe context, e.g. from the phase position of the measuring signalsupplied by a circular scanner and a jump in the OCT measuring value. Atthis transition the sign of the OCT measuring value must be flipped.

What is claimed is:
 1. A processing head for a laser processing deviceadapted for the processing of a workpiece using laser radiation, whereinthe processing head comprises; adjustable focusing optics configured tofocus the laser radiation in a focal spot having an adjustable distancefrom the processing head; an optical coherence tomograph configured tomeasure a distance between the processing head and the workpiece bymeasuring an optical interference between a first measuring light, whichwas reflected by the workpiece, and a second measuring light, which wasnot reflected by the workpiece; a path length modulator that isconfigured to change, synchronously with and dependent on a change ofthe distance of the focal spot from the processing head, an optical pathlength in an optical path along which the second measuring lightpropagates; a scanner that is arranged in the optical path of the firstmeasuring light; a scanning device, which is configured to deflect thelaser radiation in different directions; and a control device, which isconfigured to i) control a focal length of the focusing optics in such away that the focal spot is situated at a desired location on theworkpiece, ii) receive, from the coherence tomograph, informationrepresenting the distance between the processing head and the workpiece,and iii) use the information received from the coherence tomograph for acontinuous correction of a positioning of the focal spot on theworkpiece, wherein the scanner is arranged exclusively in the opticalpath of the first measuring light.
 2. The processing head of claim 1,wherein, when the focal length of the focusing optics changes by Δd, thepath length modulator changes the optical path length in the opticalpath by 2Δd.
 3. The processing head of claim 1, wherein the opticalcoherence tomograph is an FD-OCT.
 4. The processing head of claim 1,wherein the first measuring light passes through at least one movablepart of the focusing optics, and wherein the focal length of thefocusing optics depends on a position of the movable part.
 5. Theprocessing head of claim 4, wherein the first measuring light, afterpassing through the focusing optics, is always focused in the same focalplane in which the focal spot of the laser radiation is situated.
 6. Aprocessing head for a laser processing device adapted for the processingof a workpiece using laser radiation, wherein the processing headcomprises; adjustable focusing optics configured to focus the laserradiation in a focal spot having an adjustable distance from theprocessing head; an optical coherence tomograph configured to measure adistance between the processing head and the workpiece by measuring anoptical interference between a first measuring light, which wasreflected by the workpiece, and a second measuring light, which was notreflected by the workpiece; a path length modulator that is configuredto change, synchronously with and dependent on a change of the distanceof the focal spot from the processing head, an optical path length in anoptical path along which the second measuring light propagates; ascanning device, which is configured to deflect the laser radiation indifferent directions; and a control device, which is configured to i)control a focal length of the focusing optics in such a way that thefocal spot is situated at a desired location on the workpiece, ii)receive, from the coherence tomograph, information representing thedistance between the processing head and the workpiece, and iii) use theinformation received from the coherence tomograph for a continuouscorrection of a positioning of the focal spot on the workpiece, wherein,at any given time, regions of the workpiece that reflect the firstmeasuring light are not exposed to the laser radiation.
 7. Theprocessing head of claim 6, comprising a scanner that is arrangedexclusively in the optical path of the first measuring light.
 8. Theprocessing head of claim 6, wherein, when the focal length of thefocusing optics changes by Δd, the path length modulator changes theoptical path length in the optical path by 2Δd.
 9. The processing headof claim 6, wherein the optical coherence tomograph is an FD-OCT. 10.The processing head of claim 6, wherein the first measuring light passesthrough at least one movable part of the focusing optics, and whereinthe focal length of the focusing optics depends on a position of themovable part.
 11. The processing head of claim 10, wherein the firstmeasuring light, after passing through the focusing optics, is alwaysfocused in the same focal plane in which the focal spot of the laserradiation is situated.